This invention relates to bearings and, more particularly, to water lubricated sleeve bearings for use in carrying the rotating propellor shaft of water craft, as well as for use as water lubricated "steady" bearings in various industrial applications such as agitator bearings or vertical pump bearings.
Rubber, in the general sense, possesses excellent wear resistance when operated against metals under fully hydrodynamic conditions and consequently resilient rubber has been employed very successfully for such purposes. Such rubber bearings are generally made from an oil-resistant chloroprene or nitrile polymer with a Shore A hardness of between 70 and 80. Rubber is bonded to a carrying shell of bronze, brass or stainless steel and the latter is secured by mechanical means to the housing to which it is fitted. In a modified design the metallic outer sleeve has been replaced by a non-metallic phenolic casing as described in U.S. Pat. No. 3,455,613 issued July 15, 1969. This modification was carried out principally to eliminate electrolysis and facilitate removal when required. These two disadvantages of metal-sleeved bearings have also been overcome by providing the bearing with an additional sleeve of rubber molded to the outer surface of the metal element as described in U.S. Pat. No. 2,839,340, issued June 17, 1958.
Rubber bearings must, of necessity, be rather long in relation to their diameter because the relatively low modulus of elasticity of rubber used will not permit unit pressure loadings much in excess of 35 psi (6Kg/cm.sup.2). In most synthetic rubber compounds, in particular those commonly used in marine bearings, the level of hardness and consequently the compressive modulus is a function of the degree of carbon black or other filler loading in the rubber. The higher the filler loading, the harder the rubber and consequently, the higher the modulus. But this increase also carries an inverse relationship to the strength of the rubber in terms of tensile strength at break, ultimate elongation and wear resistance. The designer thus finds himself at cross purposes in the endeavour to compound an elastomer possessing good load support and high physical properties.
Problems with conventional rubber bearings also develop under certain operating conditions where full hydrodynamic lubrication does not occur. The result is a substantial squeal or howl, which prevents the use of these bearings in military craft where quiet operation is a necessity.
Because of the relatively high dry dynamic coefficient of friction of conventional rubber against metal, users are warned not to operate vessels with the bearings in a dry condition. In the inboard stern tube position a positive flow of water to conventional rubber bearings is essential to ensure a hydrodynamic film and prevent high bearing friction, temperature and rapid wear.
It is the principle object of this invention to provide a bearing capable of overcoming the shortcomings of conventional composite rubber/metal or rubber/reinforced plastic bearings.